This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-261105, filed Sep. 14, 1999, the entire contents of which are incorporated herein by reference.
This invention relates to a method for detecting the termination time point of plasma etching executed on a to-be-processed object.
An etching method using plasma is widely employed in a semiconductor manufacturing process and an LCD substrate manufacturing process. As a typical etching method, it is known to place a to-be-processed object, such as a semiconductor wafer, on a lower electrode located parallel to an upper electrode, then to apply a high frequency voltage between the electrodes, thereby discharging electricity, creating plasma of an etching gas between the electrodes and etching the object in accordance with a predetermined pattern.
In this etching method, in order to execute accurate etching, it is necessary to precisely detect an etching termination time point at which etching must be ceased. An etching termination detecting method using emission spectroscopic analysis, for example, is widely used as an etching termination detecting method. In this termination detecting method, a predetermined activated species that can be observed most easily is selected from activated species such as radicals, ions, etc. of a decomposed product or a reaction-formed product of an etching gas, and an etching termination time point is detected on the basis of variations in the intensity of emission with a predetermined wavelength due to the selected activated species. For example, when etching a silicon oxide film using a CF-based etching gas such as CF4, light of a predetermined wavelength (e.g. 483.5 nm) emitted from CO* as a reaction-formed product is detected, and the etching termination time point is determined on the basis of a changed point of the detected intensity. Alternatively, when etching a silicon nitride film using a CF-based etching gas such as CF4, light of a predetermined wavelength (e.g. 674 nm) emitted from N* as a reaction-formed product may be detected and used to detect the etching termination. Thus, in the conventional termination detecting method, light of different wavelengths is used in different etching processes.
However, in the conventional termination detecting method using emission spectroscopic analysis, the point in time is determined to be the end of etching, at which the etching of an object finishes and its underlayer is exposed, and therefore the intensity of light of a predetermined wavelength changes. Accordingly, over-etching is inevitable, with the result that the underlayer is also etched and damaged. These days, the problem of over-etching the underlayer significantly influences semiconductor products and may cause defective products, since semiconductor products are being more and more highly integrated, and accordingly, each layer including the underlayer, which constitutes the products, is made more and more thin. For example, when selectively etching a polysilicon layer as a to-be-processed layer to form a gate electrode on a gate oxide film as the underlayer of a polysilicon layer, the gate oxide film is more greatly damaged than the polysilicon layer since the former is much thinner than the latter.
It is an object of the invention to provide a method for detecting the end of etching of a to-be-processed layer without over-etching and damaging the underlayer of the to-be-processed layer.
It is another object of the invention to provide a method for detecting the end of etching of a to-be-processed layer included in a semiconductor device, without over-etching and damaging the underlayer of the to-be-processed layer, even if the semiconductor device has a multi-layer structure and hence the underlayer is very thin.
According to a first aspect of the invention, there is provided a method of detecting an etching termination time point at which a to-be-processed layer formed on an underlayer is etched using plasma, comprising:
applying, to the to-be-processed layer, at least two types of light components of different wavelengths during plasma etching, thereby causing light to reflect from the surface of the to-be-processed layer and from a boundary between the to-be-processed layer and the underlayer;
detecting those waveforms of at least two reflected light components of different wavelengths and included in the reflected light, which result from interference; and
detecting an approximate etching termination time point on the basis of a phase difference between the detected waveforms.
According to a second aspect of the invention, there is provided an etching termination time point detecting method comprising:
applying, to a to-be-processed dummy layer, light having a plurality of wavelengths, thereby causing light to reflect from the surface of the to-be-processed dummy layer and from a boundary between the to-be-processed dummy layer and an underlayer;
detecting those waveforms of at least two reflected light components of different wavelengths and included in the reflected light, which result from interference;
detecting, as an approximate etching termination time point, a time point at which a phase difference between the detected waveforms becomes 0;
setting a threshold value for detecting the approximate etching termination time point;
applying the light having the plurality of wavelengths to a to-be-processed layer included in a to-be-processed object, thereby causing light to reflect from the surface of the to-be-processed layer and from the boundary between the to-be-processed layer and an underlayer;
detecting those waveforms of at least two reflected light components of different wavelengths and included in the reflected light from the to-be-processed layer, which result from interference; and
sequentially calculating a phase difference between the detected waveforms, then comparing the calculation result with the threshold value, and determining the approximate etching termination time point of the to-be-processed object.
According to a third aspect of the invention, there is provided an etching termination time point detecting method comprising:
etching, using plasma, a to-be-processed layer on an underlayer in a low selective ratio mode in which a ratio of a rate of etching the to-be-processed layer to a rate of etching the underlayer is low;
applying, to the to-be-processed layer, at least two types of light components of different wavelengths during plasma etching, thereby causing light to reflect from the surface of the to-be-processed layer and from a boundary between the to-be-processed layer and the underlayer;
detecting those waveforms of at least two reflected light components of different wavelengths and included in the reflected light, which result from interference;
detecting an approximate etching termination time point on the basis of a phase difference between the detected waveforms; and
switching, upon detecting the approximate etching termination time point, the low selective ratio mode to a high selective ratio mode, thereby etching a remaining portion of the to-be-processed layer for a predetermined time period.
In the above methods, it is preferable that the phase difference is obtained from an intensity ratio between the detected waveforms or a differentiation value of the intensity ratio. The underlayer and the to-be-processed layer can be formed of a gate oxide film and a polysilicon layer for a gate electrode, respectively.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.